Rapid thermal processing (RTP) systems are increasingly being used for microelectronic device fabrication. As is well known to those having skill in the art, rapid thermal processing systems attain a desired processing temperature rapidly, without the need for a lengthy "ramp up" period. It has been found that rapid thermal processing systems allow microelectronic devices to be fabricated at high temperatures without causing dopant diffusion or other unwanted side effects.
In contrast with a conventional furnace which typically uses resistive heating units, a rapid thermal processing system typically uses radiant heat sources, for example arc discharge lamps or tungsten-halogen lamps. A small heating chamber is typically used, to provide a controlled environment for the wafer to be processed, and to efficiently couple the radiant heat energy from the radiant heat energy sources to the wafer. A rapid thermal processing system is described in U.S. Pat. No. 5,155,337 to Sorrell et al. entitled Method and Apparatus for Controlling Rapid Thermal Processing Systems, and assigned to the assignee of the present invention.
In semiconductor manufacturing, rapid thermal processing systems have been heretofore used for rapid thermal annealing and rapid thermal oxidation of semiconductor wafers. Attempts have recently been made to use rapid thermal processing systems for depositing layers on a semiconductor wafer, for example in chemical vapor deposition (CVD). This technique is referred to as "rapid thermal chemical vapor deposition" (RTCVD).
A known problem in rapid thermal processing systems is achieving a uniform temperature distribution across a large area wafer. Nonuniformity is typically caused by the larger surface area that exists around the wafer edge which leads to an excessive heat loss and temperature drop. See for example a publication by H. A. Lord entitled Thermal and Stress Analysis of Semiconductor Wafers in a Rapid Thermal Processing Oven, SPIE Proceedings on Rapid Isothermal Processing, Vol. 41, p. 1189 (1989) and a publication by Sorrell, Fordham, coinventor Ozturk and Wortman entitled Temperature Uniformity in RTP Furnaces, IEEE Transactions on Electron Devices, Vol. 39, p. 75 (1992).
Many solutions to the temperature nonuniformity problem have been proposed, which attempt to direct more radiation to the wafer edge or peripheral surface. See for example, U.S. patent application Ser. No. 07/953,568, now U.S. Pat. No. 5,253,324 to Wortman et al. entitled Conical Rapid Thermal Processing Apparatusand U.S. patent application Ser. No. 07/998,149 to Hauser et al. entitled Triple Heating Rapid Thermal Processing Apparatus and Method, both of which are assigned to the assignee of the present invention. These techniques have been quite successful, and RTP reactors are presently available with highly uniform temperature distributions across a large area wafer.
An additional uniformity degradation mechanism exists in rapid thermal chemical vapor deposition systems where a layer is deposited on a semiconductor wafer. The uniformity degradation mechanism is initiated by a small temperature variation across the wafer typically caused by edge cooling. The amount of light absorbed in a silicon wafer is determined by the wafer absorptivity weighted by the emission spectrum of the radiant heat lamp. The absorptivity is a strong function of the thickness and optical properties of the layers on the wafer surface. It has been found that a thin film deposition often dramatically changes the absorptivity of the wafer.
Accordingly, during rapid thermal chemical vapor deposition, once a nonuniform deposition pattern is established, the absorptivity and hence the temperature uniformity becomes strongly dependent upon the thickness uniformity. This causes the nonuniformity to increase with process time and the thickness of the deposited layer. A runaway situation therefore occurs where the absorptivity difference across the wafer continues to increase, thus resulting in greater increase of nonuniformity. This runaway condition is described in detail in a publication by the present inventors and F. Y. Sorrell entitled A Uniformity Degradation Mechanism and Rapid Thermal Chemical Vapor Deposition, Applied Physics Letters, Vol. 61, No. 22, pp. 2697-2699, November 1992, the disclosure of which is hereby incorporated herein by reference.
One thin film deposition process which is widely used in microelectronic device fabrication is deposition of a thin film of polycrystalline silicon (polysilicon) on a silicon dioxide layer on the surface of a silicon wafer. When an opaque polycrystalline silicon film is deposited on a transparent silicon dioxide layer at the silicon wafer surface, the absorptivity of the wafer changes dramatically as its surface changes.
When depositing polysilicon in an RTCVD reactor which is not capable of completely eliminating the edge cooling problem described above, a center-to-edge difference in polysilicon thickness can be established very quickly during the deposition process. In an RTCVD process, the wafer absorptivity is a function of the deposited layer thickness and therefore changes during the process. Therefore, if a thickness nonuniformity is developed on the wafer, the absorptivity becomes a function of this nonuniformity and hence varies across the wafer. Therefore, the uniformity continually degrades with increasing thickness and process time.